Vinyl chloride resins are polymers containing 50% or more of vinyl chloride. Since the vinyl chloride resins are inexpensive, are easy in hardness adjustment, and can be applied in most processing machines, they have various application fields. In addition, the vinyl chloride resins can be processed into formed products with good physical and chemical properties, and thus, are widely used in various fields.
However, the vinyl chloride resins have many disadvantages in impact strength, processability, thermal stability, and heat deflection temperature. In this respect, additives for solving these disadvantages have been developed and used. Examples of additives for the vinyl chloride resins include impact modifiers, processing aids, stabilizers, and fillers, and are selectively used according to their purposes.
Recently, there has been an increasing interest in a foam-forming process to decrease the mass of vinyl chloride resins and costs of formed products. However, foam-forming of only vinyl chloride resins cannot provide a sufficient elongation and melt strength, which causes poor appearance of formed products and low expansion ratio due to large and non-uniform foamed cells. To solve these problems, addition of a mixture of a processing aid containing methyl methacrylate as a main component and a blowing agent to a vinyl chloride resin is generally used.
U.S. Pat. No. 6,140,417 discloses a vinyl chloride resin composition including a vinyl chloride resin and a processing aid for imparting improved processability and foamability to the vinyl chloride resin. The processing aid is prepared by three-stage polymerization of a compound selected from methyl methacrylate, alkyl acrylate, and alkyl methacrylate, and a copolymerizable vinyl monomer. The three-stage polymerization includes a first stage polymerization for a low glass transition temperature (Tg), a second stage polymerization for a higher Tg than the first stage polymerization, and a third stage polymerization for a lower Tg than the second stage polymerization. U.S. Pat. No. 6,221,966 discloses a vinyl chloride resin composition including a vinyl chloride resin and a processing aid prepared by two-stage polymerization to impart improved processability, transparency, and foamability to the vinyl chloride resin. The processing aid has a small particle size, its inner layer contains a small amount of methyl methacrylate for low Tg, and its outer layer contains a large amount of methyl methacrylate for high Tg. However, these vinyl chloride resin compositions are insufficient in terms of enhancement of processability and uniformity of foamed cells.
U.S. Pat. No. 6,391,976 discloses a vinyl chloride resin composition including a small quantity of a processing aid composed of methyl methacrylate and alkyl methacrylate of 3-5 carbon atoms. However, processability and foamability are insufficient.
In addition, there have been studies about combination of functions of several additives used to overcome many disadvantages of vinyl chloride resins, i.e., multi-purpose additives. Recently, in particular, studies about additives having combined characteristics of an impact modifier and a processing aid have been done. By way of example, European Patent No. EP 1,111,001 discloses a method for improving the impact strength of a vinyl chloride resin by adding powder particles composed of impact modifier particles and processing aid particles to the vinyl chloride resin, in which the powder particles are prepared by mixing an impact modifier and a processing aid in a latex phase, followed by coagulation and drying. However, this method is restrictive since latex mixing is required. Furthermore, this method is silent about information about formation of foamed products, and thus, a sufficient impact strength is not ensured.